Prospekt MAURER Seildaempfer Englisch

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    MAURER Stay Cable Dampers

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    Vibration of Stay Cables

    Rain in combination with wind

    can cause considerable vibrations

    in stay cables. In unfavourable

    circumstances, the magnitude of

    the vibrations of the stay cables

    can endanger the total bridgestructure, in particular when the

    vibrations match the natural

    frequency of the bridge structure.

    But as a minimum, undesired

    vibrations may lead to fatigue

    problems, reducing the design

    life of stay cables, and also

    shortening the maintenance

    interval.

    Modes of vibrations of stay cables, and their natural frequencies

    Principle sketch of a cable of length L that vibrates in the 1st mode. Distance of the cable

    damper from the edge is xd, where a cable amplitude yd prevails. Maximum amplitude in

    the centre is Y1. Tension force in the cable is F, mass of the cable per meter is .

    What can be done about undesired vibrations?

    Simply speaking, when required, a so called cable vibration damper shall be attached to the

    stay cable, to reduce the amplitudes of the vibration to a tolerable level.

    Instead of the term damping, a more scientific term energy dissipation can be used. A

    damper dissipates the (undesired) energy which is introduced into the structural system. The

    source of such undesired energy can be in earth quake, or, like in this case, in a combination

    of rain and wind that excites a stay cable to vibrations that might be dangerous for the

    structural system

    What is the tolerable level of an

    amplitude?

    According to the FIB Bulletin 30/2005,

    pp.21, the maximum amplitude of a

    vibrating cable shall be kept below

    1/1700 of its length. Should theamplitude Y1 of the (undamped) cable

    exceed L/1700, a cable damper has to

    be devised.

    Fatigue of the cable

    Next to the reduction of vibrations of big

    amplitudes, another motivation for the

    installation of dampers is the reduction

    of small vibrations, which can result in

    an early ageing of the material at the

    anchor head caused by big curvatures.

    Great care has to be taken that these

    curvatures are not shifted to the locationof the damper.

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    D i s p la c e m e n t [ m m ]

    Force

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    Energy dissipation means that a certain response force which is created by the damper

    has to perform a certain displacement, and the product of force and displacement is theenergy that will be dissipated. The force could be a friction force (as we have it in the

    friction dampers) or a shear force (which is the case in elastomeric dampers), or a viscous

    response force (viscous dampers).

    The energy dissipation capacity per cycle is expressed in the hysteresis of the damper. The

    bigger the area that a hysteresis comprises, the better is its energy dissipation capacity.

    F o r c e - D i s p l a c e m e n t - D i a g r a m 0 , 2 H z - 2 0 m m

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    Hysteresis of a viscous damper (above)

    andan elastomeric damper (below),

    illustrating the superior energy

    dissipation capacity of viscous dampers.

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    Example of an externally arranged

    viscous cable damper

    (Eiland Bridge, Kampen, NL).

    Why cant we use a conventional

    damper ?

    Such shock absorbers are certainly

    cheaper than the tailor made cable

    dampers. However, their design response

    force will not match the requirement of

    the response force of cable damperswhich can reach a magnitude of 40 kN

    or more.

    Due to the fact that the optimum

    damping coefficient that is to be set is

    different in each case, an individually

    adjusted type with preset properties

    would be required, when using vehicle

    shock-dampers. In regard to delivery

    date, costs, consulting, dimensions,

    design of connections, installation,

    removal and inspection possibilities this

    is only difficult to accomplish. Long-term

    behaviour of such vehicle shock-absorbers in regard to leakage, heat

    transfer and effectiveness is also difficult

    to judge (and to guarantee). Further,

    they are no maintenance-free elements.

    What kinds of cable dampers are

    available ?

    All cable dampers have in common that

    they are attached to the stay cable close

    to their end, at the level of the bridge

    deck. At that location they will enact some

    force onto the vibrating cable, thus

    reducing the amplitude of the vibration.

    Cable dampers can be arranged internally,

    that is within the protection tube, or

    externally. Internal cable dampers are

    usually cheaper than the external ones,

    and will not affect the aesthetics of the

    bridge, and they may offer also more

    safety against vandalism or other external

    impacts. However, due to their relativecloseness to the stay cable their response

    force capacity is limited, and so is their

    damping capacity that such internal stay

    cable dampers can introduce into the

    system. This is why internal cable dampers

    have usually a limited scope of

    application, like in foot bridges, where the

    length of the stay cables is limited, and

    the damping requirement can be met with

    such internal dampers.

    Internal elastomeric Cable Damper in the test rig, showing the shear

    deformation

    External elastomeric cable damper

    installed at Forchheim Bridge

    The elastomeric cable damper

    Such a damper works by shear

    deformation of the high damping

    elastomer. Elastomeric cable

    dampers display a relatively weak

    damping characteristic, which

    mirrors the relatively low dampingcharacteristics of the elastomer.

    Elastomeric cable dampers may be

    useful for pedestrian bridges where

    the stay cables are relatively short.

    Their biggest advantage is their

    low price, and in the same time

    their weak technical performance is

    their biggest draw back.

    The friction damper

    A friction damper dissipates energy

    by means of friction forces that

    perform a displacement which

    corresponds to the amplitude of thestay cable at that location. Friction

    dampers can employ a sufficiently

    high damping ratio, however their

    functionality is limited to a

    relatively narrow range where there

    is the maximum requirement for

    damping. Below and above this

    range, a friction damper will not be

    very effective.

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    Optimum Damping

    In order to damp a certain vibration, that is a certain mode (most often the 1 stmode), we can imagine that there is a certain force acting on the cables with the

    objective to reduce the amplitudes.

    Is this force too weak, it does not create enough resistance to the vibrations,

    and so the vibrations will not decay in the optimum way. In the other extreme,

    should this force be too strong, at the location of the damper there will be no

    movement of the cable. The damper clamps the cable, and no energy will be

    dissipated. Since this point now turns to a node, the length of the cable that can

    now freely vibrate is reduced by the length where the damper clamps, and the

    cable in its reduced length can again be excited to vibrations.

    In between there is a certain force that reduces the vibrations, i.e. the

    amplitudes of the cable in an optimum way. This state is called the optimum

    damping Each modal shape has its own state of optimum damping, and the

    optimum response force of the cable damper may vary for each modal shape

    that has to be damped.

    Adaptive Cable Dampers ACDs)

    As its name implies, an adaptive linearviscous damper can adapt its response

    force to the modal shapes of the vibrating

    cable, thus making sure that always

    optimum damping is enacted. . The

    damper thus adapts to the existing

    situation on site, taking into account the

    specific parameters that make the cables

    vibrate, and tunes itself optimally to the

    situation by way of a certain algorithm

    Maurer Magneto-Rheological Dampers

    (MRDs) can be considered to be a member

    of the family of adaptive cable dampers.

    MRDs behave under a constantcurrent in

    a very good approximation like a friction

    damper. This means that the electrical

    current has to be regulated such that the

    damper force behaves in a way that the

    MR damper always works in its best

    location (which is the optimum described

    as Range 2 as explained in the section

    about friction dampers) In this best

    location the MR damper puts the

    maximum possible damping into the

    cable. This means that the MRD force

    must be controlled proportional to thevibration amplitude at the location of the

    MRD.

    Hysteresis of an adaptive magneto-

    rheological damper at different currents

    Principle sketch depicting the arrangement of the coils around the piston,

    for designing a semiactive magneto-rheological (MR) damper

    In contrast to a conventional friction

    damper: the controlled MRD prevents

    clamping (which is 1stpriority), and by

    way of an approximately optimum

    control algorithm the controlled MRD

    damps the cable in an optimum way.

    In any case, the damping effect is

    superior to the one of a conventional

    friction damper. Because, when the

    damper is off, the coupling between

    cable and bridge deck is relatively

    weak and thus vibrations of the deck

    cannot excite vibrations in the cable,

    which consequently reduces the cables

    exposure to fatigue.

    Adaptive Cable Damper (Eiland

    Bridge, Kampen, NL)

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    0 50 100 150 200 250 300

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    signal of accelerometer

    @ 24% cable length

    after double integration

    no MR damper, free cable

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    signal of accelerometer

    @ 24% cable length

    after double integration

    MR damper current I=0.00 A

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    signal of accelerometer

    @ 24% cable length

    after double integration

    MR damper current I=0.50 A

    Decay of vibrations of an undamped cable,

    i.e. no cable damper attachedMinimum decay of vibrations, when the

    attached adaptive Cable damper is in

    its base friction,i.e. no current. I = 0,

    off-mode.

    Optimum damping of an adaptive

    cable

    damper: fast decay of vibrations.

    Damper is switched on, I = 0.5 A.

    Special Features of Maurers

    Adaptive Cable Damper

    Maurer has designed the adaptive

    cable damper (ACD) on the principle

    of the magneto-rheological (MR)

    fluid damper. The fluid displays the

    characteristics that its shear strength

    can be varied under the influence of

    a magnetic field. Within the damper,

    coils are arranged which by way of

    electronic control can create a

    magnetic field.

    Decay Curves

    The effect of an adaptive magneto-rheological cable damper can be

    seen from the 3 figures above, which

    were taken from measurements at

    the Ijssel Bridge near Kampen, NL

    that were conducted by EMPA in

    Switzerland.

    The figures depict the decay of the

    amplitudes of a stay cable over the

    time, after the stay cable was excited

    by means of a rope in its natural

    frequency. Left depicts the decay of

    the undamped cable, that is without

    a cable damper. The effect of an

    inactive cable damper can be taken

    from the central figure. Inactive

    hereby refers to the state when the

    magneto-rheological damper is

    switched off, that is the current is

    0. This state which is also called the

    passive mode provides the minimum

    damping effect of an MR damper.

    The right figure above represents the

    state of optimum damping,

    Evaluating the performance of a Maurer

    adaptive cable damper (ACD) in the test

    rig of the EMPA located in Dubendorf,

    Switzerland.

    Power Supply of an adaptive

    magneto-rheological damper

    The current that is required to control

    the magnetic field in the damper

    takes a low energy supply. Should no

    fixed power supply be available, solar

    panels might suffice to provide the

    necessary energy. In the said

    example of the Ijssel Bridge near

    Kampen, NL, the power consumption

    is 21 mA when in passive mode, and

    220 mA when in active mode. This

    demonstrates that solar panels might

    very well suffice.

    The higher the electrical current, the

    stronger the magnetic field, and thehigher the damper force (see Fig. 9). The

    force of an MR damper has a lower and an

    upper limit:

    The lower limit is set by the

    base friction at 0 Ampere

    current and by the friction at

    the sealing

    The upper limit is set by the

    maximum current

    At a constant current, in good

    approximation the damper force follows

    the Coulomb Friction, this means the force

    is independent from the velocity of

    vibration. Due to the effect of base

    friction, an MR damper already employs a

    damping effect when the electric current

    is switched off, or when there is a power

    break down.

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    Reference Projects

    Sutong Bridge / China

    MAURER ACD,

    Response Force variable from 1 50 kN,

    Stroke 65 mmSupply of a total of 200 cable dampers,

    both of type ACD and passive linear

    viscous dampers.Longest cable to be damped is 588 m.

    Eilandbrug Kampen / Netherlands

    Test installation of MAURER Adaptive Cable

    Damper:

    Response Force variable from 1 40 kN,

    Stroke 60 mmLength of damped cable = 163 m.

    Bridge Dubrovnik / Croatia

    Response Force variable from 1 40 kN

    Stroke 80 mm

    Supply of 20 adaptive cable dampers (ACDs)

    Maurer Shne Head OfficeFrankfurter Ring 193, D-80807 Mnchen

    P.O. Box 44 01 45, D-80750 Mnchen

    Phone: ++49/89/32394-0

    Fax: ++49/89/32394-306

    [email protected]

    www.maurer-soehne.de

    Maurer Shne Main Branch OfficeZum Holzplatz 2, D-44536 Lnen

    P.O. Box 63 40, D-44520 Lnen

    Phone: ++49/231/43401-0

    Fax: ++49/231/43401-11

    Maurer Shne Subsidiary PlantKamenzer Str. 4 6, D-02994 Bernsdorf

    P.O. Box 55, D-02992 Bernsdorf

    Phone: ++49/35723/237-0

    Fax: ++49/35723/237-20